In the course of the battle against global warming, attentions are increasingly focused on storage devices such as, for example, lithium ion secondary batteries and electric double layer capacitors, for use in electric regeneration and output smoothing of solar batteries, wind power generations or the like, or for use as alternatives to a power itself. In consonance therewith, extensive studies have been made in various fields of industries on availability of increased output and capacity of the storage device.
Of those fields, particularly in the field of study on negative electrodes used in lithium ion secondary batteries, attentions have hitherto been centered not only on a carbon material such as, for example, graphite that is used generally as a standard material for the negative electrode, but also on a negative electrode material containing, in the active material, an element capable of increasing the capacity of the negative electrode, such as silicon or tin. Those materials for negative electrodes have a tendency to exhibit expansive behavior as they absorb lithium ions and do hence involve possibilities that particulate active materials may collapse, and that an active material layer may separate from the electrode collector layer. Since this absorption may reduce the conductivity of the negative electrode, techniques for suppressing those problems have been keenly desired for.
By way of example, Patent Document 1 listed below discloses a negative electrode or anode prepared by sintering a mixture of a particulate active material containing silicon with a conductive powdery metal on a surface of the electrode collector element under non-oxidizing atmosphere. In the practice of the preparation disclosed in this Patent Document 1, a metal foil or conductive powdery metal forming the electrode collector element is employed in the form of copper or a copper alloy.
It has, however, been found that the negative electrode prepared according to Patent Document 1 employing sintering process generates a Cu—Si compound that is electrochemically non-reactive with lithium, resulting in reduction in capacity of the negative electrode. Also, since the sintering is required to be performed at high temperature, there is a possibility that the copper used for the electrode collector element comes to melt or harden. Once such a phenomenon occurs, the flexibility required for the electrode collector element will be lost, thus posing a problem to be encountered with during the preparation of electrodes.
Patent Document 2 listed below discloses a negative electrode comprising a thin film formed on an electrode collector element, the thin film being made of a metal of a kind capable of being alloyed with lithium or an alloy containing such alloyable metal, and the electrode collector element being made of a material of a kind incapable of being alloyed with lithium. In the preparation of the negative electrode disclosed in Patent Document 2, the photoresist technique and the electroplating technique are employed to form an anode active material layer having a selective concave-convex pattern on the electrode collector element so that pores in the anode active layer formed into columnar shapes may accommodate a volumetric expansion of the anode active materials to thereby avoid an undesirable corruption of the active materials.
It has, however, been found that in order to prepare the negative electrode of the structure discussed above, preparation of a photoresist mask is needed for patterning the anode active material layer. This complicated preparation does indeed pose such a problem as to limit the productivity.
On the other hand, there has hitherto been known of an electrode element, in which a paste prepared by kneading a binder, an active material and a conductive agent together is coated on an electrode collector foil. In the preparation of this electrode, however, a sophisticated selection of a particular binder in consideration of types and characteristics of the active material and/or the conductive material are required. Further, a high level technique is also required to increase the uniformity as well as the density of the electrode enough to encounter difficulties. As a result, the characteristics of the active material have not been fully developed.
In contrast thereto, Patent Document 3 listed below suggests an electrode for use in a lithium battery, which comprises a support and an active material paste borne on the support, wherein the support is formed by depositing (or plating) a metal on a three-dimensional network plastic substrate having internal open cell by means of an arc-ion plating process, and the active material paste is a mixture of the electrode active material and a conductive agent, kneaded together with the use of a binding agent. Patent Document 3 describes the preparation of the electrode by filling or applying the active material in or to the support. Thereby, the electrode disclosed in Patent Document 3 achieves that the amount of each of the binding agent and the conductive agent to be used in preparation of the electrode can be reduced as compared with the use of a metal thin plate as an electrode substrate and that improved adhesion between the electrode substrate and the active material is effective to improve cell performance characteristics such as, for example, repeatable charge-discharge cycle, discharge electric capacity and electrical power output.
However, the plastic support containing the active material is alleged to preferably have a pore size within the range of about 50 to 100 μm. Since such plastic support must have a low density to enclose a sufficient amount of active material, the density of the electric current available from the conductive substrate tends to become low, resulting in increase in internal resistance.
Patent Document
[Patent Document 1] JP Laid-open Patent Publication No. 2002-260637
[Patent Document 2] JP Laid-open Patent Publication No. 2004-127561
[Patent Document 3] JP Laid-open Patent Publication No. H06-349481